Various electronic devices are available on the market that transform a digital or electronic image to appropriate density variations on an image carrier, in order to render the electronic image visible on the image carrier. Alternatively, the electronic image is converted to an image-wise distribution of ink repellant and ink accepting zones on a printing plate, for use in e.g. offset printing.
An electronic image is typically represented by a rectangular matrix of pixels, each having a pixel value. The location of each pixel within the matrix corresponds to a specific location on the image carrier. Each pixel value corresponds to an optical density required on the image carrier at the specific location.
In a binary system, two pixel values, e.g. 0 and 1, are sufficient, to represent a high density and a low density, which may be obtained by applying ink and no ink respectively, or toner and no toner, or generating locally dye or no dye, or by keeping and removing silver in a photographic process. In the production of printing plates, 0 may result in an ink repellant zone, where 1 results in an ink accepting zone.
In a continuous tone system, multiple density levels may be generated on the image carrier, with no perceptible quantisation to them. In order to achieve such fine quantisation, usually 256 different density levels are required, such that each pixel value may range from 0 to 255. In electrophotography usually a reduced number of density levels can be generated consistently, e.g. 16 levels, in which case the system is called a multilevel system, as opposed to a binary system or a continuous tone system.
As said before, each "zone" or "microdot" on the image carrier gets a density, corresponding to a pixel value from the electronic image. Such zone is further on indicated by the term "microdot". A microdot is the smallest space on the image carrier that can get an optical density (or ink repellency) different from neighbouring locations. Usually microdots are represented by squares or rectangles within parallel and orthogonal grid lines. The spacing of the grid lines is indicative for the resolution of the output device.
For each microdot on the image carrier, one pixel value is required. In an output device, based on image generation by exposure to light, the microdots are usually illuminated sequentially one at a time by (one or more) scanning laser beams. Microdots may be illuminated one row at a time, as with emitting LED bars or through spatial light modulators like liquid crystal (LCD) shutters or digital mirror devices (DMD).
A typical laser scanner example is the Agfa P3400 laser printer, marketed by Agfa-Gevaert N. V., which is a 400 dpi (dots per inch, one inch is 25.4 mm) printer. Each microdot has approximately a size of 62 .mu.m. The diameter of the circular spot is typically 88 .mu.m. This means that within a radius of 44 .mu.m the illuminance (W/m.sup.2) of the light beam is everywhere higher than 50% of the maximum illuminance. The illumination is usually nearly Gaussian distributed. This means that the illuminance is maximal in the centre of the microdot or in the centre of the circular spot, and decays as the distance from this centre increases. In some systems, an elliptical spot is preferred above a circular spot. Usually, the short axis of the ellipse is oriented along the fast scan direction of the laser beam, to compensate for the elongation of the illumination spot as the beam moves during the finite exposure times.
A typical LED exposure example is the Agfa P400 laser printer, marketed by Agfa-Gevaert N. V., which is equally a 400 dpi (dots per inch) printer and has an extension of the spot of typically 88 .mu.m.
In an electrophotographic system, multilevel exposure at the microdot level is used to reduce tone gradation coarseness at a given screen ruling associated with the limited addressability. Exposure intensity at the pixel level is varied and the operation point on the discharge curve is chosen such as to have a nearly linear discharge behaviour as a function of exposure for most of the exposure range used.
Because of the smooth gradation response of the photosensitive medium, for which preferentially the conductivity varies when photons impinge on its surface, e.g. an organic photoconductor (OPC), an essentially uniform energy distribution within the microdot is required. Moreover, a suitable working point of the electrophotographic process is required. This working point is characterised by parameters which are discussed in detail below.
One of the main factors to quantify the quality of a printed image is the tone scale representation, expressed by the optical density range and the exactness and stability of the contone rendering. In a digital printing machine, such as an electrophotographic engine, each tone of a contone image is produced by a certain spatial combination of some or all of the available tones per pixel. This process is referred to as screening. The set of tones, available in the machine, is defined by the properties of the exposure device. For instance, in an electrophotographic printer that uses a binary exposure device, only two tones (black and white) are available to the screening algorithm to reproduce a contone image. In some machines however, multiple tone levels are available to the screening process by applying area or intensity modulation on the output spot of the exposure device (see below). As screening is well-defined and, by its nature, perfectly repeatable, the image quality of the engine is largely determined by the ability to reproduce the set of tones. In an electrophotographic engine the contone density of each microdot is determined by the mass of toner per unit area transferred to paper. This toner mass, referred to as M/A and expressed in mg/cm.sup.2, is a function of an almost limitless amount of parameters. Most of these parameters can be regarded as fixed by design and thus invariable during the operation of the engine. Some however are extremely variable. The most important in a two-component developer system are:
toner concentration (TC)=the ratio of the amount of toner and the amount of carrier available in the developing unit in a two-component system. PA1 toner charge per unit of mass (Q/M), expressed in .mu.C/g. development potential (V.sub.DEV), expressed in Volt=the potential difference V.sub.E -V.sub.B over the development gap between the developer supply roller (bias voltage V.sub.B) and the photosensitive element (voltage after exposure V.sub.E) upon which a latent image is present. The photosensitive element is mostly implemented as an Organic Photoconductor or OPC. PA1 transfer efficiency (TE), expressed in %: the ratio of the amount of toner transferred to the printing medium and the amount of toner developed on the photosensitive element. This dependency can be formally expressed as: EQU M/A=f (TC, Q/M, V.sub.DEV, TE ) PA1 the triboelectric properties of toner and carrier, PA1 toner concentration TC, PA1 relative humidity RH of the air in the developing unit, PA1 agitation of developer in the developing unit. PA1 the initial charge level V.sub.C of the OPC, PA1 the bias voltage V.sub.B applied to the toner supply roller of the developing unit and PA1 the intensity E.sub.EXP of the image dependent illumination of the photosensitive element. PA1 toner charge Q/M, PA1 amount of toner on the photosensitive element and PA1 the value of the electric field in the transfer zone. PA1 extremely low toner charge Q/M at high relative humidity RH, leading to an increase in dust production, fogging and possibly inconsistent transfer quality over the whole tone scale. PA1 extremely high toner charge at low relative humidity, decreasing the develop ability of the toner. This requires large electric fields in the developing stage and consequently implies more powerful engine hardware.
and is generally referred to as the develop ability and transferability of the toner.
In an electrophotographic engine, the reproduction of multiple tones is highly sensitive to each of these variables. Toner concentration TC changes during engine operation due to depletion of toner caused by image development and toner addition under control of the engine. Toner charge Q/M is determined by:
When the developer is properly agitated, an unambiguous relationship can be found between Q/M, TC and RH. The development potential V.sub.DEV is determined by:
Transfer efficiency TE on its turn is, amongst other factors, determined by:
Present electrophotographic machines maintain the optical density of their produced tones by keeping toner concentration TC at a constant level. For this purpose they use a toner concentration sensor in the developing unit, or a density sensor that measures the density D.sub.OPC developed on the OPC, or both. Changes of the toner charge Q/M, due to relative humidity RH or variations of RH are compensated for by changing the development potential V.sub.DEV and the value of the transfer electric field. Disadvantages of this technique are:
Furthermore, it can be shown that for a two-component developing system, the development of the latent image is almost purely driven by toner charge Q/M. Therefore toner charge Q/M would be a valuable input to any process control system for steering the electrophotographic process. Generally, online toner charge measurement Q/M can not be implemented easily without the need for high precision measurement hardware, which leads to an increase in system variable cost. As stated before, producing several tones in an electrophotographic engine can be done by area modulation or by intensity modulation of the light beam of the exposure device (or by any combination of both). In this way, a set of microscopic tones at the pixel or microdot level are created. These form a microscopic gradation that has to be kept constant for the contone rendering, handled by the screening process, to be repeatable. The relation between the modulated output E.sub.EXP of the exposure device and the resulting development potential V.sub.DEV is extremely non-linear. Worse due to the necessary cleaning potential V.sub.CL (difference between charge potential V.sub.C and bias potential V.sub.B), there is always a range in the exposure intensity E.sub.EXP where no development potential V.sub.DEV is created. The exposure energy E.sub.EXP has to exceed a certain threshold before any development occurs. As explained above, due to changes in the develop ability of the developer (Relative Humidity RH, developer age, etc.), the development potential V.sub.DEV has to be changed in order to maintain the proper image density D. This implies changing the charge potential V.sub.C and the bias potential. By doing this, the relationship between output energy E.sub.EXP of the exposure device and the resulting development potential V.sub.DEV is altered, causing a dramatic change on the microscopic gradation D. The modulation function, used for converting tone levels I of the original image to exposure energy E.sub.EXP levels has to be redefined. In present electrophotographic machines a global linear shift and/or resealing is applied to the exposure modulation function (I, E.sub.EXP), see for instance U.S. Pat. No. 5,305,057. Because of the non-linearity and the threshold phenomenon described above, this is clearly not enough in order to maintain the highest possible contone fidelity. There is still another effect that one has to consider when producing images in a digital electrophotographic engine. Present electrophotographic engines maintain the discharge potential V.sub.E or the potential of the OPC after exposure at maximum exposure (=maximum density) at one predefined level. Changes in develop ability will require other development potentials V.sub.DEV and thus other charge potentials V.sub.C. Keeping the discharge level (E.sub.EXP).sub.MAX at the same point will consequently put the point of maximum exposure at a different point of the sensitometric curve of the OPC. The non-linear behaviour of this sensitometric curve will cause the shape of individual pixels to change dramatically due to the saturation effect. This changes individual pixel sizes and the contone rendering created by the screening process. FIG. 11 and FIG. 12 illustrate the above described effects. FIG. 11 shows two discharge curves for a typical OPC: one curve 50 for a high charge voltage V.sub.C =-440 V needed at low humidity, RH=30%, the second curve 51 for a lower charge voltage V.sub.C =-330 V needed at high humidity, RH=70%. The horizontal line 52 indicates the constant discharge potential (V.sub.E).sub.MAX for maximum exposure. On the horizontal E.sub.EXP -axis, the corresponding maximum exposure energy level E.sub.MAX for the respective humidity levels RH are found. FIG. 12 shows the resulting pixel profiles in deposited mass M/A in mg/cm.sup.2 for an exposure device with a typical gaussian spot. The maximum intensity or energy level of the spot is given by the respective E.sub.MAX values from FIG. 11. The graph 53 shows the pixel profile that corresponds with a low relative humidity RH=30%. The graph 54 shows the pixel profile that corresponds with a high relative humidity RH=70%. From the graph it is clear that the change in pixel size is not negligible.